There is widespread interest in developing new catalytic materials for large-scale chemical transformations, such as selective oxidation processes, in order to meet the global challenge of reducing energy use. The overall objective is to design materials that continuously function as catalysts at moderate operating temperature and that have high reaction selectivity for desired products. A major objective in catalysis science is to create materials that can be reproducibly prepared and that have catalytic activity and selectivity sustained over time. Although considerable advances have been made in the synthesis of exotic nanomaterials with tailored shapes, sizes and compositions, (Xia, Y. et al. Angew. Chem. Int. Ed. 2009, 48, 60; Personick, M. L. et al. J. Am. Chem. Soc. 2013, 135, 18238) there is an ongoing need to determine how to reproducibly control their catalytic behavior and enhance their longevity. Materials that blend metal compositions and bridge multiple length scales open up a wealth of opportunities to design more energy-efficient catalysts that also maximize the economical use of resources such as precious metals (Hemminger, J. et al. From Quanta to the Continuum: Opportunities for Mesoscale Science. http://science.energy.gov/bes/news-and-resources/reports). However, new materials raise fresh challenges, especially in activating these materials for catalytic process and maintaining their activity and selectivity (Hemminger, J. et al. From Quanta to the Continuum: Opportunities for Mesoscale Science. http://science.energy.gov/bes/news-and-resources/reports).
Nanoscale gold supported on metal oxides has been widely investigated as a catalyst material for selective oxidation because the relative inertness of the gold can render high selectivity (Wittstock, A. et al. Science 2010, 327, 319; Baker, T. A. et al. Phys. Chem. Chem. Phys. 2011, 13, 34; Wittstock, A. et al. Phys. Chem. Chem. Phys. 2010, 12, 12919; Haruta, M. et al. Chem. Lett. 1987, 16, 405). One important factor impeding the use of supported nanoscale gold catalysts is their propensity to agglomerate and to rapidly lose activity (Kolmakov, A. et al. Catal. Lett. 2000, 70, 93; Sykes, E. C. H. et al. J. Phys. Chem. B 2002, 106, 5390; Campbell, C. T. et al. Science 2002, 298, 811). Recently, nanoporous metal materials have been prepared using a wide variety of methods, including dealloying of bulk alloys as well as bottom-up synthetic approaches (Ding, Y. et al. Adv. Mater. 2004, 16, 1897; Erlebacher, J. et al. Nature 2001, 410, 450; Chen, L. Y. et al. ACS Catal. 2013, 3, 1220; Ma, A. et al. Sci. Rep. 2014, 4, 4849; Lee, M. N. et al. J. Phys. Chem. Lett. 2014, 5, 809; Déronzier, T. et al. J. Catal. 2014, 311, 221; Déronzier, T. et al. Chem. Mater. 2011, 23, 5287; Xu, C. et al. J. Am. Chem. Soc. 2007, 129, 42; Zielasek, V. et al. Angew. Chem. Int. Ed. 2006, 45, 8241; Lang, X. Y. et al. J. Phys. Chem. C 2010, 114, 2600; Wang, H. et al. Electroanalysis 2012, 24, 911; Lee, D. et al. J. Colloid Interface Sci. 2012, 388, 74). In particular, free-standing nanoporous Au etched from bulk Ag—Au alloys via a method that results in a dilute Ag—Au alloy (˜1-3% Ag) has been investigated as a catalyst for oxidative processes, including the selective partial oxidation of alcohols (Wittstock, A. et al. Science 2010, 327, 319; Wittstock, A. et al. Phys. Chem. Chem. Phys. 2010, 12, 12919; Xu, C. et al. J. Am. Chem. Soc. 2007, 129, 42; Zielasek, V. et al. Angew. Chem. Int. Ed. 2006, 45, 8241). Nanoporous gold sustains activity over a relatively extended period, (Xu, C. et al. J. Am. Chem. Soc. 2007, 129, 42; Zielasek, V. et al. Angew. Chem. Int. Ed. 2006, 45, 8241) most likely due to the nanoporous morphology which does not readily agglomerate at moderate temperatures. The 1-3% Ag that remains in the material after this particular etching procedure is key to the activity of nanoporous gold for oxidative catalysis. The residual Ag dissociates molecular oxygen (O2) to form adsorbed O, and the amount of Ag regulates the oxidative strength of the material (Wittstock, A. et al. Science 2010, 327, 319; Déronzier, T. et al. Chem. Mater. 2011, 23, 5287; Zielasek, V. et al. Angew. Chem. Int. Ed. 2006, 45, 8241; Moskaleva, L. V. et al. Phys. Chem. Chem. Phys. 2011, 13, 4529; Wittstock, A. et al. J. Phys. Chem. C 2009, 113, 5593). Challenges that have hindered the use of these nanoporous materials for catalysis include (1) activating them readily and (2) sustaining their activity. Current methods for activating nanoporous gold materials for catalytic partial oxidation involve flowing a mixture of reactant gases, such as CO and O2, over the catalyst at approximately 75° C. until the material becomes active for CO oxidation. This activation procedure is highly inconsistent and irreproducible (Stowers, K. J. et al. J. Catal. 2013, 308, 131); for example, some ingots of nanoporous gold activate easily, while others do not activate at all, even if kept under a steady flow of reactants for many days. In addition, nanoporous gold materials which have been activated for methanol self-coupling using this method deactivate for that same reaction after exposure to higher alcohols, such as ethanol and 1-butanol. It is of key importance to develop a reliable procedure for reproducibly activating nanoporous gold materials (Wittstock, A. at al. Science 2010, 327, 319). Accordingly, there is a need for new methods of activating gold catalysts.